Transponders, and more particularly RFID (Radio Frequency Identification) transponders, are used to obtain absolute position of vehicles, such as trains, moving along a guideway. However a transponder extends over a finite distance known as a footprint, and locating the center of a transponder (RFID) to provide precise accurate positioning information is difficult due to the nature of radio transmission propagation, specifically in tunnels.
Currently the best solutions estimate the location of the transponder center point by external means or use expensive transponders. These control the transponder footprint to ensure correct identification of the location. However, it is hard from the point of radio transmission to ensure that the footprint will not exceed its specified values.
Estimation is not good enough as it does not ensure that the train will stop accurately in a station, and specifically stations with platform doors. Expensive transponders, although they ensure that the footprint is optimized, cannot ensure the location of the transponder in a safe vital manner due to the nature of radio transmission.